1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a process for production thereof. More particularly, the present invention relates to a solid electrolytic capacitor which is substantially free from the reduction in conductivity caused by the oxidative deterioration of the conductive polymer compound used therein as a solid electrolyte, as well as to a process for production of said capacitor.
2. Description of the Related Art
There have hitherto been reported solid electrolytic capacitors as shown in FIG. 1, which comprise an anode metal 1 made of, for example, tantalum or aluminum, a dielectric oxide film 2 formed on the metal 1 by subjecting the surface of the metal 1 to anodic oxidation, a solid electrolyte layer 9 made of, for example, a metal oxide (e.g. manganese dioxide or lead dioxide) [Japanese Patent Application Kokai (Laid-Open) No. 163814/1991] or 7,7',8,8'-tetracyanoquinodimethane complex (hereinafter referred to as TCNQ complex) [Japanese Patent Application Kokai (Laid-Open) No. 79255/1977], formed on the dielectric oxide film 2, a conductive paste layer 4 made of, for example, a graphite paste or a silver paste, formed on the solid electrolyte layer 9, and a molded resin sheathing 8 (made of an epoxy resin) having all of the above members buried therein. In FIG. 1, 5a and 5b are external electrodes connected to the conductive paste layer 4 and the metal 1, respectively.
Such a solid electrolytic capacitor using manganese dioxide as the solid electrolyte layer has no sufficient conductivity and therefore shows a large impedance at a high-frequency region. A solid electrolytic capacitor using TCNQ complex as the solid electrolyte layer is interior in heat resistance because the TCNQ complex is heat-decomposed easily.
Meanwhile in recent years, as electronic apparatuses have become smaller, more speedy in response and more digitized, capacitors of good properties have come to be required.
For such a requirement, there were proposed solid electrolytic capacitors using, as the solid electrolyte layer, an aromatic conductive polymer compound (e.g. a polypyrrole) having a higher conductivity than conventional solid electrolytes (e.g. manganese dioxide, lead dioxide and TCNQ complex) and a molded resin sheathing made of an epoxy resin, a silicon resin or the like.
In these solid electrolytic capacitors using a conductive polymer compound (this undergoes oxidation easily) as the solid electrolyte, however, the oxygen in the air diffuses through the resin sheathing and penetrates deep into the capacitor in a high-temperature atmosphere, and oxidizes the conductive polymer compound. This results in lower conductivity of the conductive polymer compound and consequent increase in the high-frequency ESR (equivalent series resistance). In order to solve this problem, there were proposed, as shown in FIG. 2, a capacitor comprising an oxygen barrier layer 10 made of a resin, formed on a conductive paste layer 4 [Japanese Patent Application Kokai (Laid-Open) No. 109714/1991] and a capacitor comprising an oxygen barrier layer 10 made of a metal (e.g. solder) [Japanese Patent Application Kokai (Laid-Open) No. 109712/1991].
Also in Japanese Patent Application Kokai (Laid-Open) No. 127813/1991 and Japanese Patent Application Kokai (Laid-Open) No. 181309/1988, there were proposed processes for producing a solid electrolytic capacitor, wherein the heat treatment conducted in the steps from the formation of a conductive polymer compound layer to before the sealing with a sheathing or in the steps from after the formation of a conductive polymer layer to the sealing with a resin sheathing is conducted in an inert gas. There was also proposed, as shown in FIG. 3, a process which comprises surrounding a capacitor element 11 with an insulating frame 12 to form a gap of desired dimension between the surface of the capacitor element 11 and the insulating frame 12, filling the gap with an oxygen-free compound or a resin or fatty oil agent 13 generating no active oxygen, and then conducting sheathing [Japanese Utility Model Application Kokai (Laid-Open) No. 121918/1989].
In the processes disclosed in Japanese Patent Application Kokai (Laid-Open) No. 109714/1991 and Japanese Patent Application Kokai (Laid-Open) No. 109712/1991, however, an oxygen barrier layer (a resin layer or a metal layer) is formed in order to prevent the reduction in conductivity caused by the oxidation of conductive polymer compound layer by the oxygen in penetrating air which and this invites an increase in step. In the process disclosed in Japanese Patent Application Kokai (Laid-Open) No. 109712/1991, heating (e.g. 200.degree.-300.degree. C. in the case of solder) is conducted during the formation of metal layer or conductive polymer compound layer; this heating gives rise to dedoping phenomenon of conductive polymer compound layer, inviting reduction in conductivity of conductive polymer compound layer and increase in high-frequency ESR. The operation for capacitor production in an inert gas atmosphere, as disclosed in Japanese Patent Application Kokai (Laid-Open) No. 127813/1991 or Japanese Patent Application Kokai (Laid-Open) No. 181309/1988 must be conducted in a closed system, is inefficient, and is unsuited for mass production. The process disclosed in Japanese Utility Model Application Kokai (Laid-Open) No. 121918/1989 has drawbacks in that the firm surrounding of capacitor element with insulating frame requires a technique and the use of insulating frame incurs an increase in cost.